An optical mouse will cause a signal to be sent and prevent the monitor from shutting off. Set it and unplug the mouse. See what happens. If it shuts off, that was it.
Also, Windows Live Photo gallery screensaver will cause the monitor to stay on (just in case you're using that)
There are more possible causes as I have dealt with this quite a bit. Let me know if it was the mouse. It normally is. You may have to get another one or set it so it can't send that signal...